1. Field of the Invention
The present invention relates to a method of denitriding a high chromium molten steel with a minimum chromium loss, and more particularly to a method of denitriding a high chromium molten steel in a ladle under vacuum so as to reduce a nitrogen content in molten steel to not more than 0.0040% (40 ppm) while restraining a chronium loss to not more than 0.3%.
2. Description of the Prior Art
It is known that the presence of nitrogen in steel seriously exerts on a quality of products, and particularly in case of stainless steel containing a large amount of chromium, the reduction of nitrogen content considerably improves intergranular corrosion resistance and toughness. Recently, large-sized and thick-walled steel materials have a tendency to be used in a welded state after assembled into a constructional body by welding because it is very difficult to anneal the constructional body after the welding. Therefore, it is demanded to use the material having an extremely low nitrogen content in view of the toughness. However, the supply of low-nitrogen and tough steel materials has not yet been accomplished satisfactorily. Especially, in case of stainless steel material having a high chromium content of 25 to 35%, it is known that the nitrogen content is preferably not more than 0.0040%. However, the production of such extremely low nitrogen material is only carried out by an electron beam process under a high vacuum or a vacuum melting process using a high purity material, which have a drawback that production cost becomes very expensive.
As the method of denitriding molten steel, there is widely used a vacuum denitriding method wherein an equilibrium nitrogen content in molten steel is reduced by lowering a partial pressure of nitrogen gas in the treating atmosphere. Between the nitrogen partial pressure in the atmosphere and the nitrogen content in molten steel, there is realized an equilibrium relation as shown in the following equation according to Sievert's law: EQU N=K.sqroot.P.sub.N.sbsb.2
wherein N represents an equilibrium nitrogen content in molten steel, P.sub.N.sbsb.2 represents a nitrogen partial pressure and K represents a constant (depending upon temperature).
Usually, a pressure in the treating atmosphere or a vacuum degree realized by a ladle degassing process or the like is the order of 10.sup.-1 Torr and in this case, it is experienced that the nitrogen content in the resulting steel is higher than the equilibrium value. This is due to the fact that the actual operation is closed before the nitrogen content in molten steel does not yet reach to the equilibrium value because it is obliged to considerably shorten the vacuum treating time in the actual operation.
Therefore, it is necessary to obviate such a restriction in the actual operation that it is difficult to conduct the denitriding up to the equilibrium value in a static state. For this purpose, there have hitherto been made efforts on a process for approaching the nitrogen content to the equilibrium value as far as possible based on kinetics of denitriding reaction. For example, there is a vacuum degassing process or the like wherein an inert gas such as argon and the like is flowed into molten steel from the bottom of the ladle lined with porous refractory bricks so as to stir molten steel. In this case, it is attempted to proceed the denitriding in a short time by accelerating the following reaction formula: 2N.fwdarw.N.sub.2 (g) while using fine bubbles of the inert gas as nuclei. According to this process, however, the amount of the inert gas flowed is critical. As is well-known, the maximum limit of the flow amount is 40 Nl/min per ton of molten steel in a ladle with a usual size, for example, about 50 tons. If the flow amount exceeds the maximum limit, erosion of the porous refractory bricks is considerably promoted and also the production cost rises, so that the use of such excessive flow amount is not favorable. Therefore, the flowing has hitherto been practised by using the inert gas in an amount of not more than 40 Nl/min per ton of molten steel.
On the other hand, it is also known that when the total pressure in vacuum degassing of molten steel is maintained at 0.1 to 1 Torr in terms of a partial pressure carbon monoxide gas (Pco), even if the flow amount of inert gas is increased, there is not found a significant difference in the denitrided amount or denitriding rate, because the absolute amount of inert gas to be flowed under the above condition is still insufficient to effect the denitriding of molten steel in which N.sub.2 (g) produced by the reaction 2N.fwdarw.N.sub.2 (g) is caught by surfaces of inert gas bubbles flowed into molten steel and then removed from the surface of molten steel into the treating atmosphere. Therefore, it is necessary to use an enormous flow amount of inert gas in order to promote the denitriding reaction and considerably reduce the nitrogen content in molten steel by the vacuum degassing process with the inert gas. This is guessed to be uneconomical and has not been practised.
Recently, the production of high alloy steels such as stainless steel and the like has been performed by a vacuum decarburization process wherein decarburization refining is carried out by blowing gaseous oxygen against molten steel in a ladle under vacuum. According to this process, the decarburization is continued till a predetermined carbon content, while preventing oxidation loss of chromium to the utmost, by utilizing the feature that carbon has a strong affinity with oxygen as compared with chromium under vacuum. During the oxygen blowing, a large amount of fine carbon monoxide bubbles are produced in molten steel by the reaction C+O.fwdarw.CO (g), which contribute to considerably promote the denitriding reaction. For example, when only the oxygen gas is blown against molten steel under vacuum, the amount of CO gas evolved is usually 160 to 170 Nl/min per ton of molten steel.
Accordingly, it is apparent from the above that the amount of gas blown for promoting the denitriding of molten steel can remarkably be increased by using oxygen gas and argon gas together in the vacuum degassing process as compared with the case of flowing only argon gas into molten steel from the bottom of the ladle.
In any case, it is considered that the higher the carbon content prior to the treatment of molten steel, the more the amount of CO gas evolved and as a result, the denitriding reaction is advantageously promoted. However, when the carbon content prior to the treatment exceeds a certain predetermined value, there are caused such drawbacks that molten steel in the ladle scatters due to boiling accompanied with violent formation of CO bubbles, the decarburization time becomes long, the oxidation loss of chromium increases and the yield of steel lowers. Therefore, the carbon content prior to the treatment of molten steel has been required to be limited to not more than 0.6% until now. Considerably, the finally realized nitrogen content is 0.0070% at most, which is still unsatisfactory in view of the quality improvement.